This invention relates to a catalyst for the selective oxidation of hydrocarbons, which may or may not be saturated, to oxygen-containing organic compounds. More particularly, the invention relates to a catalyst for the selective oxidation of hydrocarbons to oxygen-containing compounds, such as the oxidation of n-butane to maleic acid anhydride. The invention further relates to a process for the selective oxidation of hydrocarbons, which may or may not be saturated, to oxygen-containing organic compounds. Finally, the invention also relates to the preparation of such a catalyst.
According to the prior art, n-butane is oxidized to maleic acid anhydride with a good selectivity in the presence of catalysts containing vanadium oxide and phosphate. Suitable catalysts can be prepared in two different manners. In both cases, the starting material is vanadium(V), whereafter the vanadium(V) is reduced to vanadium(IV), whereafter phosphate is added. According to the first method, hydrochloric acid is added to the solution of vanadium(V), whereafter at elevated temperature the vanadium is reduced with hydrogen chloride to form chlorine. According to the second method, work is done in an organic solvent, such as for instance i-butanol. At elevated temperature the organic solvent reduces the dissolved vanadium(V) which is subsequently stabilized by reaction with the dissolved phosphate.
In general the selective oxidation is carried out with an excess of oxygen. To prevent the formation of explosive mixtures, generally a content of 1.5 vol. % n-butane in air is worked with. Typical is a conversion of about 90% of the n-butane present in the feed, which is converted to maleic acid anhydride with a selectivity of about 60%. To realize such a conversion, the temperature of the catalyst bed must be maintained at about 400.degree. C. Naturally, with such an exothermic reaction the temperature in the catalyst bed will increase; generally, a higher final temperature leads to a poorer selectivity with a higher conversion. About 50% of the n-butane supplied then becomes available as maleic acid anhydride with the current technical implementation of the process.
Customarily, the oxidation reaction over the vanadium, phosphorus and oxygen-containing catalysts is carried out by passing the reactants through the reactor once, a so-called `once through` process. One of the most important limitations of this process, which uses a fixed catalyst bed, is that only an n-butane concentration of 2 vol. % at most can be used; higher n-butane contents may lead to explosions.
To increase the yield of maleic acid anhydride, recently other embodiments of the process have been proposed. One of the successful newly proposed embodiments is a fluidized bed reactor. It is then possible, without danger of explosions, to increase the concentration of the n-butane in the feed to substantially the stoichiometric value. As a result, the productivity per unit volume of the reactor increases, which leads to lower investment costs. (GB-A 2,145,010).
A fluidized bed reactor has still other advantages, viz. a markedly improved dissipation of the reaction heat, so that areas having locally a high temperature are effectively avoided. Although a fluidized bed process is economically and technically attractive, the selectivity of the catalytic oxidation is lower at higher n-butane concentrations, while moreover a (highly) wear-resistant catalyst must be used. In particular in the case of vanadium-phosphate-oxygen catalysts, the production of a wear-resistant catalyst is a difficult task.
It is known from EP-A 189,621 that the efficiency of the conversion to maleic acid anhydride can be increased by employing two separate reaction steps. The two reaction steps are carried out in separate reactors or in separate sections of a single reactor. In this case, n-butane, preferably in the absence of molecular oxygen, is contacted with the oxidized catalyst. After reaction with the lattice-oxygen present in the catalyst, whereby maleic acid anhydride is formed with a good selectivity, the residual butane and the product formed are removed, and the catalyst is passed into a reoxidation zone, where the catalyst is reoxidized with air oxygen.
Since the reduction and the oxidation of the catalyst take place in separate reactors, a more concentrated n-butane stream can be used. Moreover, in such a circulating fluidized bed, the heat transfer proceeds faster than in a fixed catalyst bed, so that the temperature is better controllable. Selectivity of up to 90% has been reported for the use of this process with two separate catalytic reactors. Although the yield of maleic acid anhydride is markedly increased, the catalyst developed for this process has two shortcomings. Because the conventional vanadium-phosphorus-oxygen catalyst is not wear-resistant, a new catalyst had to be developed, whose individual catalyst bodies are covered with a porous layer of silicon dioxide. On the one hand, this porous layer reduces wear, but, on the other hand, the inert porous layer limits the transport of reactants and reaction products. Moreover, the surface/volume ratio of the vanadium-phosphorus-oxygen constituent in the newly developed catalyst is relatively low, as in the conventional catalysts. To obtain a useful conversion of n-butane, large quantities of catalyst have to be recirculated per kilogram of maleic acid anhydride. In the literature it has been published that per kilogram of catalyst only about 2 g maleic acid anhydride is obtained.
It appears from the above-described prior art that there is a great need for a better catalyst in order to overcome the drawbacks of the current vanadium-phosphorus-oxygen. In general, wear-resistant catalysts in which the catalytically active component has a high surface area to volume ratio are obtained by providing the active component on a so-called support. Such a support is a highly porous, thermostable material, on the surface of which the active component or components are provided in more or less finely divided form. Commercially, a wide variety of preformed support bodies are available; it is therefore easy to select a suitable support with the necessary wear-resistance and a desired pore distribution. If the active component or components are provided on a support selected on the basis of the process in which the catalyst is to function, a catalyst meeting often conflicting requirements is readily obtained.
In spite of the fact that the vanadium-phosphorus-oxygen has already been employed on a technical scale for over a decade and in spite of the fact that the shortcomings of the current catalyst have been suitably recognized, efforts to prepare a satisfactory supported vanadium-phosphorus-oxygen catalyst have been unsuccessful to date. The selectivity of the supported vanadium-phosphorus-oxygen catalysts prepared heretofore was invariably found to be unacceptably low.
This is the reason that alternatives have been searched for. The first possibility has already been mentioned above, viz. the provision of a porous layer of wear-resistant silicon dioxide on porous particles of the vanadium-phosphorus-oxygen catalyst. Another method of preparing an improved catalyst is described in the above-mentioned GB-A 2,145,010. There a mixture of the oxides of vanadium and phosphorus is treated with an acid, preferably phosphoric acid or hydrochloric acid, the material thus obtained is mixed with zirconium dioxide or titanium dioxide, and the suspension obtained is subsequently spray-dried. Wear-resistant bodies of dimensions of from 3 to 10 .mu.m are then obtained. It will be clear that neither of the methods leads to catalysts where the active component is provided on (highly) porous support bodies in finely divided form.
An important disadvantage of the known catalysts based on vanadium, phosphorus and oxygen (VPO) is, moreover, the requirement that the catalysts must possess a specific structure to obtain the desired activity and selectivity.
A first object of the present invention is to provide a suitable catalyst for the selective oxidation of hydrocarbons, which catalyst is less dependent on the structure of the active component and which moreover requires no special measures for increasing its usefulness in various more modern oxidation processes.